In dual-fuel combustion systems for gas turbines, gaseous and liquid fuel are used separately to fire the gas turbine. For a number of reasons, including the goal of dry low No.sub.x operations, developmental emphasis has been on the efficient use of gas fuel nozzles and the liquid fuel nozzle is typically used only as a backup and only sporadically. However, it has been demonstrated recently that the use of liquid fuel in the gas turbine combustor in lieu of gaseous fuel has a tendency to deposit carbon residue on various passages of the gas/air fuel nozzle which may inhibit return to gas fuel turbine operation.
More particularly, catastrophic combustor failure has resulted from the rapid build-up of carbon deposits in the vicinity of the liquid fuel injection. Carbon deposits developed during liquid fuel operation tend to block off the intended gas or liquid fuel inlet to the combustor liner, causing a backflow of fuel. This results in ignition and flame holding external to the combustor liner, which in turn results in thermal failure. Because the failure does not result in loss of flame and may not result in significant exhaust temperature changes prior to breaching the combustion pressure vessel, these failures can become non-contained prior to control system detection. Serious safety concerns have thus been generated and load restrictions have been required to limit operation of the turbine and hence combustors in load ranges where deposition rates are known to be significant. For example, some failures have been reported following less than 12 hours of operation at adverse high carbon formation rate conditions.
Various efforts to eliminate or accommodate the carbon deposition problem have been proposed. For example, aerodynamic sweeping of surfaces anticipated to experience liquid fuel impingement has been demonstrated to be effective where there is sufficient combustor line pressure drop to create enough air sweep velocity to inhibit deposition. Combustor surface metal temperatures in excess, for example, of 800.degree. F., may also prevent carbon formation. Both of these systems, however, have their respective drawbacks, including substantial usage of inlet air which could be more advantageously used elsewhere, as well as metal cooling problems. Consequently, there has been a demonstrated need to provide a dual-fuel nozzle system of fuel spray distribution and combustor aerodynamic characteristics that minimize or prevent initiation of carbon deposition on air, liquid fuel and gaseous fuel nozzle surfaces. One such dual-fuel nozzle system is disclosed in U.S. Pat. No. 5,833,141, of common assignee herewith. The present disclosure constitutes an improvement upon and a variation of the system disclosed in that prior application.